The mechanical and chemical modes which are used by nettles to 'sting' have been studied for more than 150 years.

The toxic effects of extracts from stinging hairs of Urtica spp. have not been well characterized due to the difficulty in extracting toxins effectively from the hairs."

[source link below]

It's believed that the silica-based hairs release a cocktail of toxic chemicals including oxalic acid, formic acid, tartaric acid, acetylcholine, histamine, and serotonin. But calculations regarding the amounts of injected chemicals suggest that irritation should be mild and only last a few minutes. In fact the irritation and swelling usually persists for a few hours, and tingling sensations in the affected area can last for more than 12 hours.

Stinging hairs, although studied for a long time, are still mysterious, particularly concerning the mechanism of the skin reaction after being stung. In the current studies, the newly identified pain-inducing agents, oxalic acid and tartaric acid, are two ubiquitous components of terrestrial plant tissues. Tartaric acid has been suggested as a component of Urtica (Thurston and Lersten, 1969), but oxalate was excluded as a component of U. ferox (Pilgrim, 1959). Their role in plants and the mechanism underlying the induced pain are issues which remain to be fully addressed."

Source : Annals of Botany, Volume 98, Issue 1, 1 July 2006